1. Field of the Invention
The present invention relates to a method and device for the projection printing of a mask onto a workpiece, particularly onto a semiconductor substrate being coated with a photoresist.
2. Description of the Prior Art
In the production of intigrated circuits masks having different circuit patterns are imaged on the photoresist of a semiconductor substrate. After development, the photoresist serves for covering the substrate at desired points during subsequent processing cycles, e.g. etching- and diffusing processes. As very high demands to accuracy are made in the manufacture of integrated circuits, e.g. line widths in the range of 1 .mu.m have to be imaged, projection lenses having a reduction factor of 10, for example, are used for imaging the mask pattern onto the substrate. Due to various marginal conditions, such as the size of the image field and the available types of glass, lens systems being transparent to wavelengths &gt;400 nm are used for the projection lens. Moreover, the lens has to be color-corrected in order to avoid errors due to chromatic aberration. Said color correction can only be made in the case of band widths of about 6 nm.
The spectral sensitivity of the employed photoresists is the determining factor for the choice of suitable light sources. The most frequently used photoresists fulfilling the requirements in respect of adhesion, workability, resolution and etching resistance contain diazo compounds as photosensitve components. The spectral sensitivity of photoresists reaches a maximum in the range of wavelengths of about 400 nm and drops rapidly in the range to about 450 nm.
A mercury vapor lamp is mostly used as light source emitting spectral lines having wave lengths of 405 nm and 436 nm. The spectral line at 436 nm has most frequently been used up to the present for exposing the photoresist, as said line is of higher intensity. The color correction of the objective lens has been made accordingly. It is, however, of disadvantage that the absorption curve of photoresists drops in the range of 436 nm. Consequently, a slight displacement of said absorption curve, which could occur during the manufacture and application of the photoresist, causes considerable changes of the correct exposure time. Changes of only a few percents have already adverse effects. Moreover, the resolution at an exposure wave length of 436 nm is worse compared with wave length of 400 nm, which can still be transmitted by projection lenses.